Noise is always a consideration in dynamic random access memory, DRAM, design and is a major factor in causing invalid data. As the size of the DRAM array increases and more memory cells are added to form DRAMs of enormous storage capacity (for instance 16 mega bit DRAMs where more than 16 million bits of data may be stored on a single memory chip) the voltage lines that supply voltage to the array become lengthy, causing designers to be ever more concerned with noise considerations. Noise crossing a voltage line can cause the voltage on the line to swing. This can cause invalid data to be read from the memory array and can cause invalid data to be written to the memory array.
Some VLSI circuits use regulated voltages in their design and operation. It is believed that the use of regulated voltages add to the robustness of the overall VLSI design in controlling noise. It is known that regulated voltages may be supplied to a VLSI device from an external power supply. It is also known that regulated voltages may be generated on the VLSI chip itself by using voltage regulator circuits.
In the case of VLSI DRAM design, it has been discovered that it is desirable to use regulated voltages because they increase the noise margin of the device. However, problems arise in designing a suitable voltage regulator circuit. For instance, in VLSI DRAM design, the load on the output voltage of the voltage regulator circuit is large and varying due to many different transistors of the memory array switching on and off as the array inputs and outputs data. The switching of the transistors generates noise. Noise and loading can cause the output regulated voltage to become unstable thereby tending to become unsuitable for its intended purpose. A device is therefore needed to buffer the stable reference voltage.
Previous buffer devices have been found to be unsuitable. A push-pull type output driver is not suitable when the output voltage is close to one of the supply voltages due to threshold voltage loss on either output devices. A classic class AB type driver is not suitable because the standby current increases when the supply voltage increases. (It is known that for TTL logic signals, the supply voltage is designed for +5v, however, due to noise, loading, and other tactors, the supply voltage typically swings between +4v and +6v.) Even with the addition of local feedback, the classic class AB driver does not respond unless the output voltage swings large enough. Additionally, the output stage has to be biased with some DC current. This makes it even more difficult to do output compensation. Although it offers better stability control, a single stage high gain buffer is not suitable because of the limit on both the input and output voltage ranges. Two stage buffers usually call for Miller compensation that uses a standard capacitor that is unavailable to most digital processes.
It is desirable therefore to have a buffer with output compensation that can be effectively used in the voltage regulator design of a VLSI DRAM. Ideally, the buffer should dissipate a constant current over supply variations while maintaining the ability to swing in response to output voltage variations. It should supply current drive and be able to both source and sink current at its output. It should be able to respond to output voltage noise.
It is an object of this invention to provide a buffer that provides output voltage close to its voltage supply.
It is a further object of this invention to provide a buffer that dissipates small and constant DC power over a wide range of voltage supply.
It is a further object of this invention to provide a buffer that provides both sourcing and sinking currents responsive to output changes.
It is a further object of this invention to provide a buffer that uses output compensation for stabilization.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.